This invention relates to means to indicate the application of predetermined stress-strain loads to driven, headed and screw-threaded fastenings. In many applications, it is desirable to accurately determine the load level being placed on a bolt, screw, nut or other fastener. Examples of such applications include refineries, chemical plants, other heavy structural work, internal combustion engine cylinder head bolts, and bolts supporting mine roofs or ceilings.
In the past, the loading on a fastener has often been measured by devices such as torque wrenches or hydraulic bolt tensioners. Other methods of tightening a bolt to a predetermined load require heating the bolt to a calculated expansion before running down the nut or tightening the bolt manually and measuring the elongation of the bolt with a micrometer. This is called heat cycling. All of these methods depend on sophisticated tools and gauges, most of which are not suitable for field use. Torque wrenches lack sufficient accuracy because the actual loading on the bolt is not necessarily that indicated by the torque wrench because of tight, rough or dirty threads, because of dirt from the flat faces subject to relative rotation in tightening the nut, because of variations in the properties of any lubricant used, and because of wide variations in the coefficient of friction in the threads.
Various other devices for indicating the loading on bolts have been suggested, which devices fit between the fastener head or nut and a bearing surface. Examples of such devices include a wedged motion translator which converts the small axial movements into a circumferential movement, increasing the circumfernce of a split ring, which increase is measurable on two attached radially extending arms (Adise U.S. Pat. No. 3,060,731); a washer which collapses or buckles when the compressive load reaches a predetermined level (Lewis U.S. Pat. No. 3,174,386); a dynomometer which converts the strain on a washer-like element into a measurable electrical resistance (Ramberg U.S. Pat. No. 2,493,029); and an assembly having cantilevered flags which project outwardly between the bolt head and the bearing surface and which are extruded by a bearing shoe and deflect downwardly when the stress on the bolt becomes excessive (Harrison U.S. Pat. No. 3,104,645).
It is also known that conical or belleville spring washers may be placed between the fastener head or nut and the bearing surface and that the compressive deflection of such washers from their normal conical shape to a substantially flat state varies in an approximate direct relation to the load being placed upon the fastener (Knocke U.S. Pat. No. 2,781,687 and Ralston U.S. Pat. No. 2,850,937). However, the amount of the deflection of such washers is very small even with large compressive loads. To accurately determine the proper load on the fastener, the deflection of the washer must be accurately measured, for example, as within 0.001 inches. Such accurate measurement requires special tools which are not adaptable for field use.
Various devices have been suggested for use in conjunction with belleville or conical spring washers to assist in the measurement of the compressive deflection of such washers. Most of these devices include a triggering mechanism which is activated when a predetermined compressive load has been placed on the washer (Setzler U.S. Pat. No. 3,474,701; Rumbough U.S. Pat. No. 3,736,394; Curtis U.S. Pat. No. 3,881,392). With each of these devices the manufacturing costs are generally high, the device may not be reusable, the visual indications are relatively small, and the accuracy of the device is limited by the action of the triggering mechanism so that the device is not necessarily as accurate as the measured deflection of the spring washer.